This invention relates to high-temperature, secondary electrochemical cells and batteries of such cells that can be employed as power sources for electric automobiles, storage of energy generated during intervals of off-peak power consumption and various other applications. It is particuarly applicable to electrochemical cells that employ metal sulfides as positive electrode active materials and lithium alloys such as lithium-aluminum or lithium-silicon as the negative electrode active material.
A substantial amount of work has been done in development of these types of electrochemical cells and their electrodes. Examples of such high-temperature cells and their various components are disclosed in U.S. Pat. No. 3,887,396 to Walsh et al., entitled "Modular Electrochemical Cell", June 3, 1975; U.S. Pat. No. 3,907,589 to Gay and Martino, entitled "Cathodes for a Secondary Electrochemical Cell", Sept. 23, 1975; U.S. Pat. No. 3,933,520 to Gay and Martino entitled "Method of Preparing Electrodes with Porous Current Collector Structures and Solid Reactants for Secondary Electrochemical Cells", Jan. 20, 1976; and allowed U.S. Pat. No. 3,941,612 Mar. 2, 1976 to Steunenberg et al., entitled "Improved Cathode Composition for Electrochemical Cell". The method of the present application is in particular an improvement to that disclosed in allowed U.S. Pat. No. 3,947,291 Mar. 30, 1976 to Yao and Walsh, entitled "Electrochemical Cell Assembled in Discharged State", filed Sept. 30, 1974. Each of these patents and patent applications is assigned to the assignee of the present application.
Previous methods for preparing electrochemical cells in uncharged state have been limited in the cell capacity that can be provided in small and light-weight cells. In these earlier procedures, lithium sulfide and an electrolytic salt such as lithium chloride-potassium chloride eutectic were repetitively ground and heated to a temperature in excess of the melting point of the electrolyte to produce a powdered mass of finely divided lithium sulfide particles coated and wetted with electrolyte. The amount of lithium sulfide that could be included within this electrode material was limited by the consistency of the mixture which became increasingly viscous with lithium sulfide content.
The electrode material thus prepared is pressed into the openings within a layer or a stack of iron transition metal mesh to form the positive electrode. On charging of this cell, the iron or other transition metal within the mesh corrodes and reacts with the lithium sulfide to form a metallic sulfide such as FeS which serves as the positive electrode reactant. This type system requires not only the corrosion but the diffusion of the iron through the electrode material in order to obtain a uniform composition. Positive electrodes prepared by this procedure have been found to have low initial capacities and require several break-in cycles. In addition, only thin electrodes prepared in this manner have performed well, thus making the preparation of high-capacity cells difficult or impractical. The use of thin electrodes with large areas in cells having practical quantities of active material for commercial cells can result in nonuniform current distribution and high diffusional polarization.
Therefore, in view of these shortcomings in prior art methods, it is an object of the present invention to provide a method of preparing an electrochemical cell in uncharged state with increased capacity.
It is a further object to provide a simplified method for preparing a positive electrode in uncharged state with minimum grinding and heating steps.
It is also an object to provide a method of preparing a positive electrode in uncharged state that will exhibit near maximum capacity in early cycles.